I keep a running list of ideas, some of them are specific to a particular spot (e.g. a rose on my kneecap), some are just "zone specific", as in I want it on my leg somewhere, and will decide based on the drawing and where the artist thinks it works best. I am always doing research and looking for references.
Lately I've been trying to figure out what to do for larger scale work, like my ribs or back - I have some basic ideas, but it's harder to come up with things to fill it out, like background composition, how to handle the boundaries, things like that. Obviously I will need to sit down with a tattooer to figure that stuff out.
On the other hand, I also enjoy doing last-minute/walk-in type stuff. Recently I went and got something off a sheet of halloween flash, which was super fun. A few years back, I went in for a quick touch-up, was just hanging out at the shop for a while, and ended up getting something else quick too.